June 30, 2004

Large
galaxy clusters represent the largest gravitationally stable assemblies
of matter in the universe. Massive clusters of galaxies consist of
thousands of galaxies and vast quantities of extremely hot
intergalactic gas all held together by gravity. It is believed that
clusters of galaxies grow in size and in number through time and we see
many more galaxy clusters in the nearby (recent) universe than in the
distant past. The assembly and growth of massive clusters involve
complex interactions between dark matter, diffuse gas, and thousands of
individual galaxies. The physical details of these interactions are
currently not fully understood.

Courtesty of G. L. Bryan & M. L. Norman

Figure 1:
The "cosmic web" predicted by numerical simulations of formation of
structures in the universe. Galaxy clusters would correspond to the red
regions.

Both
computer simulations and theoretical work predict that the growth
process of massive clusters is highly non-isotropic with the in-fall of
matter and galaxy mergers occurring along preferred directions. The
coalescence and funneling of matter should produce highly correlated
structures in space, enhancing visibility in these preferred
directions. This is referred to the “cosmic web” (Figure 1).
Such structures look like highways converging onto a large city, but
with the important difference that clusters feeding filaments are
organized in three-dimensional space.

Figure 2:
V, R and z' color image of 20 x 20 arcmin2 region around the cluster
MACS J0717.5+3745 as obtained with SupremeCam on the Subaru Telescope.
The contours map the different levels of galaxy overdensity compared to
the background. The high density contour region (top right) corresponds
to the cluster and the shallower region in the bottom left part
corresponds to the filament.

Using
the entire battery of large telescopes on Mauna Kea, Harald Ebelling,
Elizabeth Barrett and David Donovan of the Institute for Astronomy
(IfA) at the University of Hawaii (UH), have obtained a unique data set
of the X-ray cluster MACS J0717.5+3745 and its surroundings. MACS
J0717.5+3745 is one of the most massive clusters of galaxies known at a
redshift greater than z=0.5. The UH team obtained wide-field images in
the V, R, I and z’ photometric bands with SuprimeCam on the Subaru
Telescope, and multi-object spectroscopy with LRIS & Deimos on Keck
and GMOS on Gemini North. From these data, they present evidence of a
spectacular large-scale filament along which matter and galaxies is
being funneled into the cluster. This appears to be a generic filament
as opposed to a merger event.

The image of the region (Figure 2)
reveals an apparently coherent over-density of red galaxies greater
than 6.3 Megaparsec (Mpc) in length, of which about 4.3 Mpc can be
attributed to a long filament structure. The red color of the galaxies
tracing the filament means that they are old systems very similar to
the ones found in the cluster core. This has important implications for
models trying to discern between different physical mechanisms that
might be responsible for the transformation from blue spiral galaxies
in the field to red elliptical galaxies in the cores of galaxy clusters.

The filament extends well beyond the virial radius
(~2.3 Mpc) of the cluster, (the virial radius defines the physical size
of the gravitationally stable part of the cluster). The observed
elongated morphology is indicative of significant dynamical activity at
scales greater than 5 Mpc from the cluster, well beyond the virial radius.
The observations imply an in-fall of matter along this preferred axis
direction. The funneling will persist for roughly the next 4 Gyr,
assuming an in-fall speed of ~1000 km/s.

Figure 3:
This figure shows the locations of all galaxies with spectroscopic
redshifts. Filled symbols mark galaxies with redshifts between 0.52 and
0.57 (i.e. within 3 sigma range of the systemic redshift of the
cluster). The dotted line marks a circle of 1 Mpc radius around the
nominal cluster center.

This
is the first convincing candidate for the type of filament channeling
of matter onto massive clusters predicted by numerical simulations.
Colors of the individual galaxies and velocities derived from the
GMOS/LRIS/DEIMOS spectra do confirm that the over-density contours (Figure 2) maps galaxies at the cluster redshift (Figure 3). The entire filament is located at z =0.55.

The
observed galaxy distribution in and around MACS J07017.5+3745 supports
a picture in which massive clusters of galaxies grow via discrete as
well as continuous in-fall of matter along large-scale filaments.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Maunakea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in five partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, the Brazilian Ministério da Ciência, Tecnologia e Inovação and the Chilean Comisión Nacional de Investigación Científica y Tecnológica (CONICYT). The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.